1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, and more particularly, it relates to technique of isolating adjacent element regions by isolation regions.
2. Description of the Prior Art
FIGS. 2A to 2G are sectional views showing principal manufacturing steps for forming element regions and isolation regions by a conventional oxide film isolation method. With reference to FIGS. 2A to 2G, description is now made on the conventional method.
First, as shown in FIG. 2A, a nitride film 201 and an oxide film 101 having a thickness of about 100 nm are successively stacked on a p.sup.- -type semiconductor substrate 1 of low impurity concentration. Resist films 301 are formed in regions substantially corresponding to isolation regions on the nitride film 201 to perform etching by utilizing the resist films 301 as masks, thereby to leave two-layer films only on predetermined regions of the p.sup.- -type semiconductor substrate 1, as shown in FIG. 2B. Symbol X denotes first regions in which the two-layer films of the oxide film 101 and the nitride film 201 are left and symbol Y denotes second regions from which the oxide and nitride films 101 and 201 are removed. A plurality of such second regions Y are defined to be surrounded by the first regions X.
Then the two-layer films of the oxide film 101 and the nitride film 201 are utilized as masks to introduce an n-type impurity such as arsenic into the second regions Y of the semiconductor substrate 1 through ion implantation or the like, thereby to form n.sup.+ -type impurity implantation layers 21 as shown in FIG. 2B.
Thereafter the nitride films 201 are utilized as masks to perform selective oxidation, thereby to form oxide films 102 of several hundred nm in thickness on the second regions Y of the semiconductor substrate 1. The oxide films 102 are preferably three to ten times the thickness of the oxide films 101. Then, heat treatment is performed to diffuse the n-type impurity contained in the n.sup.+ -type impurity implantation layers 21, thereby to form n.sup.+ -type buried layers 22 of high impurity concentration as shown in FIG. 2C.
Then the nitride films 201 are removed by etching and the oxide films 102 are utilized as masks to introduce p-type impurity such as boron into the first regions X of the semiconductor substrate 1 through the oxide films 101 by ion implantation or the like. Thereafter heat treatment is performed to diffuse the p-type impurity, thereby to form p.sup.+ -type buried layers 3 of high impurity concentration between adjacent pairs of the n.sup.+ -type buried layers 22 in FIG. 2D.
The oxide films 101 and 102 are removed by wet etching through hydrofluoric acid (HF), to form n.sup.- -type epitaxial layer 4 of low impurity concentration containing n-type impurity such as arsenic on the semiconductor substrate 1 provided on its surface part with the n.sup.+ -type buried layers 22 and the p.sup.+ -type buried layers 3, as shown in FIG. 2E.
A nitride film 202 and an oxide film 103 having a thickness of about several ten nm are successively stacked on the surface of the n.sup.- -type epitaxial layer 4, to form resist films 302 in regions corresponding to element regions on the nitride film 202. Then the resist films 302 are utilized as masks to remove a two-layer film of the oxide film 103 and the nitride film 202 by etching, thereby leaving the two-layer films only in the regions corresponding to the element regions. Then, three-layer films of the oxide films 103, the nitride films 202 and the resist films 302 left as shown in FIG. 2F are utilized as masks to etch regions of the epitaxial layer 4 corresponding to isolation regions to prescribed depth.
Then the resist films 302 are removed and selective oxidation is performed by utilizing the nitride films 202 as maskd, thereby to form thick oxide films 104 in regions corresponding to the isolation regions as shown in FIG. 2G. The oxide films 104 are formed to be partially in contact with the n.sup.+ -type buried layers 22 in the bottom sides thereof as shown in FIG. 2G. Finally the nitride films 202 and the oxide films 103 are removed.
In the conventional method of manufacturing a semiconductor device through the oxide film isolation method, the regions for introducing the p-type impurity into the p.sup.- -type semiconductor substrate 1 (regions corresponding to the oxide films 101 of the p.sup.- -type semiconductor substrate 1 in FIG. 2C) are determined by the oxide films 102 formed on the regions in which the n-type impurity has been introduced into the p.sup.- -type semiconductor substrate 1 in the preceding step (regions corresponding to the n.sup.+ -type impurity implantation layers 21 in FIG. 2B), whereby edges of the regions introduced with p-type impurity and those introduced with the n-type impurity are substantially at the same positions.
Therefore, when extreme floating diffuaion of the p.sup.+ -type impurity takes place during formation of the n.sup.- -type epitaxial layer 4 after formation of the p.sup.+ -type buried layers 3 and the n.sup.+ -type buried layers 22, p-type regions 33 are partially left in the n.sup.- -type epitaxial layers 4 serving as the element regions in a later step of forming the oxide films 104 for isolation, as shown in FIG. 2G.
When the p-type regions 33 are thus partially left in the n.sup.- -type epitaxial layers 4, a plurality of p-type diffusion layers to be formed in the n.sup.- -type expitaxial layers 4 in a later step are brought into contact with the p-type regions 33, to be electrically coupled with each other through the p-type regions 33.
The isolation regions may be expanded in consideration of the residual p-type regions 33, whereas such expansion of the isolation regions prevents implementation of the semiconductor device with high density of integration.
The aforementioned problem is also caused when the p-type and n-type polarities are inverted in the manufacturing steps as shown in FIGS. 2A to 2G.